I could reproduce this and it seems that it is a bug in the HtmlPlaceholder.
Your Telerik Points have been updated for the feedback.
The problem seems to arise from the fact that Layout is not updated for the HtmlPlaceholder. For me this happened only when I explicitly set the size of the Placeholder, otherwise it was disappearing as expected.
The workaround here is to update the visibility of the control if you are switching to the tab with this control, like so:
|void tabControl_SelectionChanged(object sender, RoutedEventArgs e)
| //let's say that the HtmlPlaceholedr is on the second page (index 1)
| placeholder.Visibility = tabControl.SelectedIndex == 1 ? Visibility.Visible : Visibility.Collapsed;
Does this work for you?
the Telerik team